Customized transition zone system and method for an ablation pattern

ABSTRACT

The present invention discloses a corneal refractive procedure providing a customized transition zone. The customized transition zone provided according to the methods and systems of the present invention exhibits continuous curvature between an ablated optical zone and a non-ablated zone to address curvature discontinuity at the edge of the optical zone, thereby minimizing the biomechanical response and its postoperative effects on vision.

The present invention relates generally to eye surgery, and moreparticularly to a corneal refractive procedure providing a customizedtransition zone, which exhibits continuous curvature between an ablatedoptical zone and a non-ablated zone to address curvature discontinuityat the edge of the optical zone, thereby minimizing the biomechamcalresponse and its post-operative effects on vision.

Approximately 60% of Americans have refractive errors, and millions ofpeople are myopic worldwide. Thousands of laser refractive surgeries areperformed every year for the correction of myopia. Of the manyindividuals treated, about 15% to 50% do not achieve 20/20 vision due,at least in part, to the relationship of the patient eye to the meanpopulation response eye, the dependence of refractive procedures on themean population response eye, and ablation patterns that do not takeadvantage of the widest possible ablation zone achievablebiomechanically. Additionally, many individuals cannot benefit fromconventional corneal ablative techniques because their eyes do not fallwithin parameters modeled by the mean population response eye. Forexample, using conventional techniques, some corneas are not thickenough for a desired correction.

Early attempts at photorefractive keratectomy (PRK) modeled the corneaas two refracting surfaces with a bulk material in between the tworefracting surfaces where there was a known index of refraction. Intreating myopia, the goal was to increase the anterior radius ofcurvature, thus decreasing the curvature of the anterior surface. Asimple geometric formula resulted, which assumes that the targetedcorneal shape was a function of the ablation profile. This is the “shapesubtraction” paradigm which assumes that the final corneal shape isdetermined by how much tissue is subtracted (ablated) by a laser.Essentially, this model treats the cornea as a piece of plastic to besculpted into an ideal surface shape by laser ablation.

For example, FIG. 1 is a schematic of the simple “shape-subtraction”paradigm for correcting myopia where the cornea is initially “toocurved” and thus is conventionally “reshaped” towards a desired“flatter” profile. R₁ and R₂ are initial and final radii of curvature,respectively, d is the maximum ablation depth, and S is the diameter ofthe optical zone. In some conventional processes, the starting point indetermining how much material is to be removed from the center of thecornea to change its curvature is determined by the following geometricformula, the Munnerlyn formula: d=R₂−sqrt(R₁ ²−h²)−R₂+sqrt(R₂ ²−h²),where R₁−sqrt(R₁ ²−h²) is the distance of center front from origin, andR2 is the new desired radius of curvature. In other conventionalprocesses, the starting point within the optical zone is anapproximation of the Munnerlyn formula, which says that d=DS²/3 where Dis the amount of desired diopter treatment and S is the optical zone,which is equal to 2 h. As typically used, this approximation calls for a12-micron depth ablation of corneal tissue per diopter of treatment overa 6.0 mm chord treatment zone.

FIG. 2 is another schematic diagram of the shape-subtraction model ofrefractive surgery for a myopic ablation. The pre-operative radius ofcurvature is R₁ and the desired post-operative curvature is R₂. Theintervening tissue between the pre-operative curve (solid) andpost-operative curve (dashed) is “subtracted” with an excimer laser toproduce the desired result. Thus, conventionally, corrections arelimited by the amount and/or character of ablation that can occur withinthe optical zone. The shape subtraction model assumes that the onlyportion of the cornea that is changed during an ablative procedure isthe area within the ablation zone and that even if there are changesoutside the ablation zone; they have no effect on central vision. Thus,ablation patterns conventionally do not account for the correctiveeffect outside the optical zone caused by the biomechanical response ofthe cornea to the operative procedure. While the shape subtraction modelhas yielded satisfactory results without considering the biomechanicalresponse of the cornea to perturbation, it can be improved.

Additionally, the conventional reshaping in the conventional opticalzone of a cornea has unanticipated and undesired results. By way ofillustration shown in FIG. 3, the edge of optical zone S where theablated cornea meets the non-ablated cornea is a both a slope and acurvature discontinuity (i.e., points where the first and secondderivatives along a programmed contour are step functions), whichresults in various visual problems including, but not limited to,spherical aberration, glare and halo effects. These effects can beparticularly acute in low light conditions when the pupil enlarges. Itis to be appreciated that the larger the difference between R₁ and R₂,the greater the slop and curvature discontinuities.

Conventionally, a transition zone is added to ablation patterns used inregular procedures and “customized” procedures (e.g., those based ontopographical and/or wave-front analyses). As shown by FIG. 4, thetransition zone is defined as the area between programmed optical zoneS₁ and the ablation zone S₂ used for the corrective procedure. Forexample, a one or two millimeter wide transition zone is added to aconventional (e.g., six millimeter) optical zone for a total ablationpattern of around eight millimeters.

Conventional transition zones are typically linear or stepwise reductionpatterns sized to be completed in the allocated transition zone width.Although such conventional transition zones reduce the abrupt change inslope between the “programmed” optical zone (i.e., area of corneaablated for corrected measures) and non-ablated cornea in an attempt tominimize the above-mentioned visual problems, they are not thought tocontribute to the corrective effect and improved image qualityanticipated by ablation in the optical zone. Additionally, suchconventional transition zones still have a curvature discontinuity andhigh curvature that generates a spherical aberration. Furthermore, thecorrective effect of the transition zone on vision is not conventionallyaccounted for in conventional or customized procedures. Moreover, bothregular conventional procedures and customized conventional proceduresare limited by the amount of available cornea to ablate. Conventionally,the wider the optical zone is made, the deeper is the depth of ablation.Understandably, there is a finite amount of corneal depth to ablate, andthus, with conventional patterns, a finite ablatable width. For example,FIG. 5 illustrates two sample depths for two sample optical zone widths.To illustrate, for a desired ten or twelve diopter myopic correction,with a 500 micron thick cornea, conventional patterns may be limited toa 4.5 mm optical zone before running out of ablatable cornea. For adesired correction, 4.5 mm may not be wide enough even with a 1.5 mmconventional transition zone, and induce glare, and/or halos and/orspherical aberration. Thus, conventional methods may not produce thedesired correction.

Accordingly, there is room for more improvement based on the methods andsystems described herein.

In one embodiment, a corneal ablative pattern based on individualbiomechanical responses to cutting, ablating and/or peeling a cornea isdisclosed. The corneal ablative pattern includes a wider correction zonethat includes both an optical zone and a transition zone, where thetransition zone has a continuous curvature and its effects on visioncorrection are accounted for in the pattern design. The individualbiomechanical responses can be predicted from pre-operative measurementsand/or from measurements taken during surgery such as for example, aftera flap cut during laser in-situ keratomileusis (LASIK). The individualbiomechanical responses can depend on significant differences in thematerial properties of living human corneas and thus, there aresignificant differences in the responses of various individuals tosimilar ablative profiles.

Since a flap is cut in a LASIK procedure, the biomechanical responses ofthe cornea to the cut can be employed to predict a biomechanicalresponse to ablation. Thus, measurements taken before and after cuttingthe flap facilitate adapting parameters for subsequent ablative steps.The prediction can be facilitated by reference to a biomechanicalresponse model that relates inputs to outputs in light of a plurality ofrelationships between corneal measurements and individual responses thatlead to post ablative corneal shape. In procedures like those where noflap is cut, such as for example, LASEK and PRK, predictions aboutbiomechanical responses can still be made based on pre-operativemeasurements and a biomechanical response model. In another embodiment,a method for improving a transition zone is disclosed. The methodcomprises measuring a peripheral curvature of a pre-operative cornea,and developing an ablation depth profile in a transition zone, where theablation depth profile will produce a continuous curvature on thesurface of a post-operative cornea, where the curvature will becontinuous through the transition zone.

In still another embodiment, a system for customizing an ablativepattern design with an improved ablation zone comprising both an ablatedoptical zone and an ablated transitional zone of continuous curvature isdisclosed. The system comprises means for acquiring a pre-operative databy measuring a cornea pre-operatively, and means for acquiring apost-perturbation data by measuring a cornea after a perturbation hasbeen applied to the cornea. The system further includes means foracquiring a prediction data of the biomechanical response of a cornea toablation based, at least in part, on the pre-operative data and thepost-perturbation data, and means for designing an ablation patternbased, at least in part, on the prediction data, where the ablationpattern considers the corrective effect of a transition zone.

In still another embodiment a method for customizing a transition zonefor a refractive ophthalmic treatment is disclosed. The method comprisesmeasuring a curvature of a pre-operative cornea. The method furtherincludes developing an ablation depth profile in a transition zone,where the ablation depth profile will produce a continuous curvature onthe surface of a post-operative cornea, where the curvature will becontinuous throughout the transition zone thereby minimizing curvaturediscontinuities.

In yet another embodiment, a system for customizing a transition zone ofan ablation pattern for a refractive ophthalmic treatment for a corneais disclosed. The system comprises a data receiver for receiving acorneal data. The system further includes a transition zone designeradapted to produce the customized transition zone with a continuouscurvature which eliminates curvature discontinuities at or near the edgeof a post-operative optical zone and whose effects minimizes thebiomechanical response in the post-operative cornea.

In another embodiment, a method to facilitate an increased functionaloptical zone with a customized transition zone pattern of continuouscurvature, where the corrective properties of the transition zone areincluded in the ablation zone pattern design is disclosed. The methodcomprises receiving pre-operative data concerning a cornea on which arefractive ophthalmic treatment will be performed, and subtracting theprogrammed optical zone correction from corneal measurements provided inthe pre-operative data to provide location of the programmed opticalzone edge. The method further includes calculating the predictedcurvature at and/or near the edge of the optical zone after applicationof the programmed optical zone correction, and calculating based, atleast in part, on the pre-operative data received and the predictedcurvature at the edge, a customized transition zone pattern. Thecustomized transition zone addresses curvature discontinuity byeliminating its occurrence in and/or near the programmed optical zone.

The method also includes applying the calculated transition zone to theablation zone pattern.

These and other features and advantages of the invention will be morefully understood from the following description of preferred embodimentsof the invention taken together with the accompanying drawings. It isnoted that the scope of the claims is defined by the recitations thereinand not by the specific discussion of features and advantages set forthin the present description.

The following detailed description of the embodiments of the presentinvention can be best understood when read in conjunction with thefollowing drawings, where like structure is indicated with likereference numerals and in which:

FIG. 1 illustrates a shape subtraction model.

FIG. 2 illustrates a shape subtraction model of refractive surgery formyopic ablation.

FIG. 3 illustrates a transition between an ablated area and anon-ablated area having curvature discontinuities.

FIG. 4 illustrates a conventional transition zone between an ablatedarea and a non-ablated area.

FIG. 5 illustrates two different ablation widths and the correspondingablation depths.

FIG. 6 illustrates a conceptual model for predicting biomechanicalcentral flattening due to severing corneal lamellae.

FIG. 7 illustrates a conceptual model of an ablation pattern comprisinga customized transition zone modifying a programmed optical zonecorrection according to the present invention.

FIG. 8 is a schematic block diagram of an illustrative computingenvironment that supports the methods and systems described herein.

FIG. 9 is a flow chart of an example method for designing a customizedtransition zone pattern.

FIG. 10 is a schematic block diagram of an example system for designinga customized transition zone pattern.

FIG. 11 illustrates a simulated tangential map and a simulatedrefractive power map for a cornea before ablative surgery.

FIG. 12 illustrates a simulated tangential map and a simulatedrefractive power map for a cornea following a conventional ablativeprocedure.

FIG. 13 illustrates a simulated tangential map and a simulatedrefractive power map for a cornea following an ablative procedure thatemploys the systems and methods described herein for an improvedtransition zone.

Skilled artisans appreciate that elements in the figures are illustratedfor simplicity and clarity and have not necessarily been drawn to scale.For example, the dimensions of some of the elements in the figures maybe exaggerated relative to other elements to help to improveunderstanding of embodiment(s) of the present invention.

The corneal ablative pattern described herein provides a wider ablationzone that includes both an optical zone and a transition zone, where thetransition zone has a continuous curvature and its effects on visioncorrection are accounted for in the pattern design. It is to beappreciated that post-ablative corneal shape, and thus visualperformance, is the function of at least three factors: the ablationprofile, the healing process, and the biomechanical response of thecornea to structural change. Furthermore, there are only certain shapesa cornea will biomechanically assume. These shapes depend, at least inpart, on epithelial thickness, stromal thickness and response tosevering stroma and/or lamellae. For example, the deeper the ablation inthe edge of the ablation zone in a myopic procedure to generate apotentially desirable post-operative prolate shape, the greater thenumber of severed lamellae and the greater the biomechanical centralflattening response to counter the effect. The transition zone of thepresent invention considers such factors.

LASIK, LASEK, and/or PRK procedures can be improved by takingpre-operative measurements of the eye, predicting the cornea'sbiomechanical response to ablative treatment, and customizing anablative pattern design. Additionally, wave-front guided procedures mayalso employ a transition zone that may benefit from the systems andmethods described herein. Parameters considered in customizing theablation pattern design with an improved transition zone include, butare not limited to, the location, size, shape, depth, and number of cutsand/or ablations. In addition to pre-operative measurements, real timemeasurements taken during the initial steps of a surgical procedurefacilitate analyzing individualized responses and thus in designing acombined ablation zone that includes both an optical zone and animproved transition zone of continuous curvature, and which considersand accounts for the corrective effects of the conventional transitionzone. For example, comparison of data including, but not limited to,pre-flap, post-flap, pre-ablation and post-ablation data like cornealthickness, flap thickness, corneal topography, and wave-front dataprovide predictive information applicable to modifying an ablationpattern design.

As used herein, the term “biomechanical response” means a mechanical orphysical response to a perturbation or other stimulus (e.g., cutting thecornea, ablating the cornea).

As used in this application, the term “computer component” refers tocomputer-related entities such as hardware, firmware, software, acombination thereof, or software in execution. For example, a computercomponent can be, but is not limited to being, a process running on aprocessor, a processor, an object, an executable, a thread of execution,a program and a computer. By way of illustration, both an applicationrunning on a server and the server can be computer components. One ormore computer components can reside within a process and/or thread ofexecution and a computer component can be localized on one computerand/or distributed between two or more computers.

“Signal”, as used herein, includes but is not limited to one or moreelectrical or optical signals, analog or digital, one or more computerinstructions, a bit or bit stream, or the like.

“Software”, as used herein, includes but is not limited to, one or morecomputer readable and/or executable instructions that cause a computeror other electronic device to perform functions, actions and/or behavein a desired manner. The instructions may be embodied in various formslike routines, algorithms, modules, methods, threads, and/or programs.Software may also be implemented in a variety of executable and/orloadable forms including, but not limited to, a stand-alone program, afunction call (local and/or remote), a servelet, an applet, instructionsstored in a memory, part of an operating system or browser, and thelike. It is to be appreciated that the computer readable and/orexecutable instructions can be located in one computer component and/ordistributed between two or more communicating, co-operating, and/orparallel processing computer components and thus can be loaded and/orexecuted in serial, parallel, massively parallel and other manners.

In a myopic procedure, inside the ablation zone, thickness decreases aspredicted by the shape subtraction model. However, outside the ablationzone, thickness unexpectedly increases. In addition, regression analysisbetween the central and peripheral curvature changes shows a negativecorrelation (p<0.0053), indicating that greater central flatteningproduces greater peripheral steepening. Elevation and pachymetry mapsalso show peripheral increases in both elevation and pachymetry outsidethe ablation zone, corresponding to the increase in curvature. Onceagain, the shape subtraction model is found lacking, and thus ablativedesign patterns based on the shape subtraction model can be improved byconsidering individual biomechanical response to corneal structuralchanges and contribution of areas outside the optical zone to centralvision.

Altering the corneal structure alters the shape of the entire cornea,whether using an incisional, ablative or thermal mechanism.Fundamentally, if the cornea were a piece of plastic, radial keratotomywould not work. Yet, in laser refractive surgery, the structural linkbetween the central and peripheral cornea has not been employed todesign ablation patterns. Thus, this application describes systems andmethods that consider peripheral stromal thickening or an increase inthe corneal elevation outside the optical and/or ablation zone whendesigning an ablation pattern. Regression analysis between centralcurvature change and peripheral elevation change from thirty subjectswho underwent LASIK procedures demonstrated a positive correlation(R2=0.56, p<0.0001) indicating that the greater the increase inelevation outside the ablation zone, the greater the flatteningcurvature change centrally. In one case study, regression analysis ofcentral curvature versus peripheral stromal thickness was performed.Central curvature has a negative correlation with peripheral thickness,both inferior and superior, meaning the greater the peripheralthickness, the flatter the central curvature. Thus, the applicationdescribes example systems and methods that design ablation patterns withimproved transitional zones with continuous curvature based, at least inpart, on peripheral stromal thickness.

An example model is presented in FIG. 6 that predicts biomechanicalcentral corneal flattening as a direct consequence of severed corneallamellae. Rather than modeled like a piece of plastic as with priormethod, the cornea in the present invention is modeled as a series ofstacked rubber bands (lamellae) with sponges between each layer(interlamellaer spaces filled with ground substance or matrix). Thelamellae carry a tensile load since there is a force pushing on themfrom underneath (intraocular pressure) and the ends are held tightly bythe limbus. The amount of water that each matrix can hold is determinedby how tightly the lamellae are pulled. As the lamellae are pulled moretightly, tension increases and water is squeezed out of the interleavingmatrix resulting in smaller, interlamellaer spacing. This is analogousto the pre-operative condition illustrated in FIG. 6.

After myopic laser refractive surgery, a series of lamellae are severedcircumferentially and removed centrally as shown in FIG. 6. Theremaining peripheral lamellae segments relax just as tight rubber bandswould relax once cut. With the reduction of tension in the lamellae, thesqueezing force on the matrix is reduced and the distance betweenlamellae expands due to negative intrastromal fluid pressure. This isanalogous to the sponges taking up water if the rubber bands are cut.This allows the periphery of the cornea to thicken. Due to cross linkingbetween the lamellae layers, the expansion force pulls on the underlyingintact lamellae as indicated by the arrows pointing radially outward. Anoutward force in the periphery pulls laterally on the center andflattens it. Thus, the cornea will flatten centrally with proceduresthat circumferentially sever lamellae, including hyperopic proceduresand therapeutic procedures.

The biomechanical flattening enhances a myopic procedure, works againsta hyperopic procedure, and causes flattening in a non-refractive PTK.This includes myopic profiles, hyperopic profiles, constant depth PTKprofiles, as well as the simple cutting of a LASIK flap. Thus, theexample systems and methods described herein rely on the assumption ofcentral corneal flattening and peripheral steepening associated withsevering lamellae. FIG. 7 is an illustration of a customized transitionzone calculated according to methodology of the present invention. Asshown in this illustrated example, the transition zone t_(z) modifiesthe ablation pattern A starting at the programmed optical zone S toeliminate curvature discontinuities at the edge of optical zone S. It isto be appreciated that a transition zone t_(z) based at least on thepre-operative data could also conceivable start at the edge or inside ofthe programmed optical zone S. It is to be appreciated that by expandingthe ablation pattern A and widening and/or deepening the transition zonet_(z) (i.e., the programmed optical zone correction depth), cornealcrosslinks, which are preferentially distributed anteriorly andperipherally, are removed. Computational modeling indicates thatapplying such a transition zone pattern reduces the biomechanicalresponse of the cornea and allows a more optimal corneal shape to beachieved.

In example models employed by the systems and methods described herein,the stroma, which makes up about 90% of the total corneal thickness,most significantly influences the mechanical response of the cornea toperturbation (e.g., cutting, ablation). The stroma is approximately 78%water by weight, 15% collagen, and 7% other proteins, proteoglycans, andsalts. Three hundred to five hundred lamellae, flattened bundles ofparallel collagen fibrils, run from limbus to limbus withoutinterruption. In the posterior two thirds of the stroma, the lamellaeare successively stacked parallel to the corneal surface so that eachlamellae has an angular offset from its anterior and posteriorneighbors. Anteriorly, the lamellae are more randomly oriented, oftenobliquely to the corneal surface, are more branched, and aresignificantly interwoven. Accounting for the biomechanical response oflamellar severing in the expanded transition zone facilitates improvingresulting vision.

The systems and methods described herein assume that shape changesmeasured outside an optical zone and/or ablation zone can affectcurvature changes within an optical and/or ablation zone. Centralcurvature change in refractive surgery is not solely a product of theoptical zone design pattern. When peripheral thickness and ablation zonebias are included in a regression model, over 83% of the variance andcurvature response is explained by peripheral thickening. Thus, abiomechanical response considered herein assumes additional flatteningover and above conventionally programmed ablation profiles. This occursin myopic (with an intent to flatten), hyperopic (with an attempt tosteepen), and/or non-refractive PTK.

In LASIK surgery, cutting the flap alters the corneal structure.Following the cutting of the corneal flap, corneal measurements takenare therefore employed by example methods and systems described herein.The microkeratomic incision for the flap produces changes in the cornea.The redistribution of strain caused by the keratomic incision causes thecentral cornea to flatten and the peripheral stroma matrix to thickenand become steeper. Such reshaping assists with a central myopiccorrection, where decreased corneal curvature is prescribed, and worksagainst a hyperopic correction where increased corneal curvature isprescribed. Since cutting the LASIK flap produces a biomechanicalresponse, a method for customizing a refractive ophthalmic treatment bydesigning the ablation pattern with an improved optical zone plustransitional zone can include pre-operatively measuring the cornea,cutting the flap, measuring the cornea and/or the flap, calculating acustomized transition zone for a designed ablation pattern, andperforming an ablation based on the pattern.

In LASEK and PRK surgery, no flap is cut. However, based onpre-operative measurements, with reference to a biomechanical responsemodel, an ablation pattern with an improved optical zone plus transitionzone is similarly provided.

In an example method, corneal measurements are taken by methodsincluding, but not limited to, corneal topography, optical coherencetomography, ultrasound, refraction, and/or wave-front analysis. InLASIK, these measurements are taken before and after the microkeratomicincision for the corneal flap. In LASEK and PRK, the measurements areonly taken before the procedure. In LASIK, differences in cornealmeasurements are compared to expected and achieved post-cut results.Comparison of the measurements before and after the incision facilitateadjusting an ablative pattern design to account for the measuredbiomechanical response due to the cutting, and the predictedbiomechanical response anticipated through the ablation. Ablationpattern adjustments can thus be made in advance of the ablation in aseparate procedure and/or in real time as an intraoperative adjustmentafter the perturbation (e.g., cut, ablation) but before the ablation.

Optimal surgical procedures benefit from considering the biomechanicalresults of the ablative procedure itself. Thus, an example LASIKsurgical technique employing the systems and methods described hereinincludes, but is not limited to, accessing a model of predictedresponses based on empirical data collected from corneas before andafter cutting a flap, ablation, and healing. The technique furtherincludes taking pre-perturbation measurements (e.g., thickness profile,curvature profile, corneal size) and employing these measurements inconnection with the model to facilitate computing ablation patterndesigns. The method further includes taking post perturbationmeasurements (e.g., thickness profile, curvature profile, centralflattening, peripheral thickening), and employing these measurements tocompute one or more ablation parameters. A sample LASEK or PRK surgicaltechnique would only acquire pre-operative data that can be employed toquery the biomechanical response model. The technique further includescomparing the differences between the pre-perturbation measurements andthe post perturbation measurements to facilitate refining the computedablation parameters and/or the model.

Example biomechanical response models can also consider Young's modulusmeasurements, and other factors including, but not limited to, age, sex,race, years of contact lens wear, thickness, curvature, and cornealsize. While data concerning some factors are acquired by measuring,other data can be acquired during a patient interview e.g., years ofcontact lens use). Measurements of one or more of these factorsfacilitate selecting which pre-operative parameters to employ to predicta biomechanical response. Thus, ablation patterns may be designed basedon pre-operative data.

Pre-operative and postoperative measurements can be input to one or morecomputer components by methods including, but not limited to, keystroke, direct data transfer, and so on. During cornea surgery, themethods described herein may be performed on a computer system withwhich a surgical team member communicates. Data may be input to thecomputer during the surgical process. The method can then produce data,such as ablation parameters, that can be employed in subsequent steps ofthe surgery.

Those skilled in the art of computer programming, mathematical computermodeling, and/or data base manipulation and administration will readilyappreciate that example systems and methods described herein may beembodied in software and/or one or more computer components. Forexample, FIG. 8 illustrates a computer 800 that includes a processor802, a memory 804, a disk 806, input/output ports 810, and a networkinterface 812 operably connected by a bus 808. Executable components ofsystems described herein may be located on a computer like computer 800.Similarly, computer executable methods described herein may be performedon a computer like computer 800. It is to be appreciated that othercomputers may also be employed with the systems and methods describedherein. The processor 802 can be a variety of various processorsincluding dual microprocessor and other multi-processor architectures.

The memory 804 can include volatile memory and/or non-volatile memory.The non-volatile memory can include, but is not limited to, read onlymemory (ROM), programmable read only memory (PROM), electricallyprogrammable read only memory (EPROM), electrically erasableprogrammable read only memory (EEPROM), and the like. Volatile memorycan include, for example, random access memory (RAM), synchronous RAM(SRAM), dynamic RAM (DRAM), synchronous DRAM (SDRAM), double data rateSDRAM (DR SDRAM), and direct RAM bus RAM (DRRAM). The disk 806 caninclude, but is not limited to, devices like a magnetic disk drive, afloppy disk drive, a tape drive, a Zip drive, a flash memory card,and/or a memory stick. Furthermore, the disk 806 can include opticaldrives like, compact disk ROM (CD-ROM), a CD recordable drive (CD-Rdrive), a CD rewriteable drive (CD-RW drive) and/or a digital versatileROM drive (DVD ROM). The memory 804 can store processes 814 and/or data816, for example. The disk 806 and/or memory 804 can store an operatingsystem that controls and allocates resources of the computer 800.

The bus 808 can be a single internal bus interconnect architectureand/or other bus architectures. The bus 808 can be of a variety of typesincluding, but not limited to, a memory bus or memory controller, aperipheral bus or external bus, and/or a local bus. The local bus can beof varieties including, but not limited to, an industrial standardarchitecture (ISA) bus, a microchannel architecture (MSA) bus, anextended ISA (EISA) bus, a peripheral component interconnect (PCI) bus,a universal serial (USB) bus, and a small computer systems interface(SCSI) bus.

The computer 800 interacts with input/output devices 818 viainput/output ports 810. Input/output devices 818 can include, but arenot limited to, a keyboard, a microphone, a pointing and selectiondevice, cameras, video cards, displays, and the like. The input/outputports 810 can include but are not limited to, serial ports, parallelports, and USB ports.

The computer 800 can operate in a network environment and thus isconnected to a network 820 by a network interface 812. Through thenetwork 820, the computer 800 may be logically connected to a remotecomputer 822. The network 820 includes, but is not limited to, localarea networks (LAN), wide area networks (WAN), and other networks. Thenetwork interface 812 can connect to local area network technologiesincluding, but not limited to, fiber distributed data interface (FDDI),copper distributed data interface (CDDI), ethernet/IEEE 802.3, tokenring/IEEE 802.5, and the like. Similarly, the network interface 812 canconnect to wide area network technologies including, but not limited to,point to point links, and circuit switching networks like integratedservices digital networks (ISDN), packet switching networks, and digitalsubscriber lines (DSL).

In view of the exemplary systems shown and described herein, examplemethodologies that are implemented will be better appreciated withreference to the flow diagram of FIGS. 9 and 10. While for purposes ofsimplicity of explanation, the illustrated methodologies are shown anddescribed as a series of blocks, it is to be appreciated that themethodologies are not limited by the order of the blocks, as some blockscan occur in different orders and/or concurrently with other blocks fromthat shown and described. Moreover, less than all the illustrated blocksmay be required to implement an example methodology. Furthermore,additional and/or alternative methodologies can employ additional, notillustrated blocks. In one example, methodologies are implemented ascomputer executable instructions and/or operations, stored on computerreadable media. The computer readable media includes, but not limitedto, an application specific integrated circuit (ASIC), a compact disc(CD), a digital versatile disk (DVD), a random access memory (RAM), aread only memory (ROM), a programmable read only memory (PROM), anelectronically erasable programmable read only memory (EEPROM), a disk,a carrier wave, and a memory stick.

In the flow diagrams, rectangular blocks denote “processing blocks” thatmay be implemented, for example, in software. Similarly, thediamond-shaped blocks denote “decision blocks” or “flow control blocks”that may also be implemented, for example, in software. Alternatively,and/or additionally, the processing and decision blocks can beimplemented in functionally equivalent circuits like a digital signalprocessor (DSP), an application specific integrated circuit (ASIC), andthe like.

A flow diagram does not depict syntax for any particular programminglanguage, methodology, or style (e.g., procedural, object-oriented).Rather, a flow diagram illustrates functional information one skilled inthe art may employ to program software, design circuits, and so on. Itis to be appreciated that in some examples, program elements liketemporary variables, routine loops, and so on are not shown.

Turning now to FIG. 9, a flow chart illustrates an example method 900that facilitates an increased functional optical zone with a customizedtransition zone pattern of continuous curvature, where the correctiveproperties of the transition zone are included in the ablation zonepattern design. At 910, the method 900 receives pre-operative dataconcerning a cornea on which a refractive ophthalmic treatment will beperformed. The pre-operative data in part is used to determine aprogrammed optical zone correction used in the ablation zone pattern.The pre-operative data can include, but is not limited to, topographicdata, pachymetric data, elevation data, corneal thickness data, cornealcurvature data, wave-front data, and intraocular pressure data, wheresuch data are associated with the cornea before it has been perturbed.The perturbation can be one of a corneal incision, a corneal ablation, aLASIK flap cut, ultrasonic measurements, and peeling the epitheliallayer from the cornea, for example. It is to be appreciated that theablation zone pattern comprises the programmed optical zone correction(pattern) and the customized transition pattern derived from themethodology of the present invention.

At 920, the method 900 subtracts the programmed optical zone correctionfrom corneal measurements provided in the pre-operative data to providelocation of the programmed optical zone edge. At 930, the method 900calculates the predicted curvature at and/or near the edge of theoptical zone after application of the programmed optical zonecorrection. Optionally, at 935, the method 900 may receivepost-perturbation data that can be used to correlate and/or recalculatedthe predicted curvature at and/or near edge of the optical zone forfurther refinement.

At 940, based, at least in part, on the pre-operative data received at910 and the predicted curvature at the edge calculated at 930, themethod 900 then calculates a customized transition zone pattern whichaddresses curvature discontinuity by eliminating its occurrence inand/or near the programmed optical zone. In particular, with thepredicated curvature at and/or near the edge of the optical zonecalculated at 930, the method 900 uses a curve fitting algorithm whichgenerates a transition zone with a continuous second derivative alongthe profile of the cornea outwardly from the programmed optical zonecorrection. It is to be appreciated that any conventional curve fittingalgorithm, such as spline fitting, arc-step fitting, least-squaresfitting, non-linear least squares fitting may be used by the presentinvention.

At 950, the method 950 applies the calculated transition zone to adesigned ablation pattern. The ablation pattern may be designed inaccordance with a biomechanical response modeled in a biomechanicalresponse model, such as discussed in co-pending application entitledParametric Model Based Ablative Surgical System and Methods, Ser. No.______, commonly assigned to The Ohio State University, which theteachings of which is herein incorporated fully be reference. In oneexample, the biomechanical response model predicts the biomechanicalresponse, at least in part, by considering the impact of severingcorneal lamellae during the perturbation.

In an extension of method 900 (not illustrated), additional processingmay be undertaken. This additional processing includes receiving postperturbation data, which can include, but is not limited to, topographicdata, pachymetric data, elevation data, total corneal thickness data,corneal curvature data, wave-front data, flap thickness data, andintraocular pressure data. The perturbation can be, for example, acorneal incision, a corneal ablation, a LASIK flap cut, and anepithelial layer peel. The additional processing can also includereceiving post-operative diagnosis data and selectively updating thebiomechanical response model based, at least in part, on thepost-perturbation data and/or the post-operative diagnosis data. In thisway, a predictive biomechanical response model can be updated over timeto become more complete and thus provide predictions that are even moreaccurate and thus better designs. The post-operative diagnosis data caninclude, but is not limited to, patient satisfaction data, patientvision data, patient halo effect data, topographic data, pachymetricdata, elevation data, total corneal thickness data, corneal curvaturedata, wave-front data, and intraocular pressure data.

Turning now to FIG. 10, a system 1000 for calculating a customizedtransition zone, which addresses curvature discontinuities in aprogrammed optical zone correction, is illustrated. The system 1000includes a data receiver 1010 that receives corneal data 1020. Thecorneal data 1020 can include, but is not limited to, topographic data,pachymetric data, elevation data, total corneal thickness data, cornealcurvature data, wave-front data, and intraocular pressure data measuredbefore and/or after a cornea is processed by at least one of a cut, anablation, and an epithelial peel. In LASEK and PRK, the corneal datawill not include post-perturbation data, with the design being based onpre-operative measurements with reference to a biomechanical responsemodel, such for example, as discussed in co-pending application entitledParametric Model Based Ablative Surgical System and Methods, Ser. No.______, commonly assigned to The Ohio State University, which theteachings of which is herein incorporated fully be reference.

The system 1000 may also optionally include a predictive cornealcurvature model 1030. The predictive corneal curvature model 1030facilitates selecting a transition zone curvature based on the cornealdata 1020. The system 1000 includes a transition zone designer 1040 thatcomputes the transition zone curvature pattern 1050. The designer 1040produces a customized transition zone pattern of continuous curvaturewhich eliminates curvature discontinuities at or near the edge of thepost-operative optical zone and whose effects on minimizing thebiomechanical response can then be applied and accounted for in anablation zone design.

Turning now to FIG. 11, a simulated tangential map and a simulatedrefractive power map associated with a theoretical pre-operative corneaare illustrated. On the tangential map warmer colors (e.g., yellow, red)represent more curved areas. On the refractive map, the warmer colorsrepresent areas of greater power. Conversely, cooler colors (blue,purple) represent areas of lesser curvature on the tangential map, andlower power on the refractive map. Thus, in FIG. 11, the tangential mapillustrates that the cornea is steeper in the middle and flatter in theperiphery, thereby graphically illustrating a typical myopic cornea.

FIG. 12 illustrates a simulated tangential map and a simulatedrefractive power map for a theoretical post-operative cornea, where thedata set used represents a cornea ablated using a conventionaltransition zone and ablation algorithm. The tangential map illustrates acentral flattening as compared to the pre-operative cornea of FIG. 11,and the characteristic red ring that indicates a very steep area causedby a sub-optimal transition zone design and peripheral biomechanicalresponse. The refractive power map has a characteristically smallcentral blue area that can be improved using the systems and methodsdescribed herein.

FIG. 13 illustrates a tangential map and a refractive power map for atheoretical post-operative cornea, where the data set used represents acornea ablated using the customized transition zone described herein inan ablation pattern. In both FIGS. 12 and 13, the resulting theoreticalpost-operative corneas, which had substantially the same optical zonesize and central flattening, applied programmed corrections based on theMunnerlyn formulae. However, unlike that of the post-operative cornea ofFIG. 12, the tangential map of FIG. 13 illustrates that the red ringindicating an area of curvature discontinuities has been moved furtheraway from the center of the cornea. This facilitates mitigating problemsassociated with spherical aberration. In addition, by applying thecustomized transition zone, a larger area of improved curvature can beachieved. For example, in the refractive power map of FIG. 13, thecentral blue area is much larger than that of FIG. 12, indicating alarger, more useful area of corrected vision, with fewer effects ofspherical aberration.

What has been described above includes several examples. It is, ofcourse, not possible to describe every conceivable combination ofcomponents or methodologies for purposes of describing the methods,systems, computer readable media and so on employed in improving thecomputation of ablation parameters. However, one of ordinary skill inthe art may recognize that further combinations and permutations arepossible. Accordingly, this application is intended to embracealterations, modifications, and variations that fall within the scope ofthe appended claims. Furthermore, to the extent that the term “includes”is employed in the detailed description or the claims, it is intended tobe inclusive in a manner similar to the term “comprising” as that termis interpreted when employed as a transitional word in a claim.

1. A method for customizing a transition zone for a refractive ophthalmic treatment comprising: measuring a curvature of a pre-operative cornea; developing a programmed ablation depth profile in a transition zone, where the programmed ablation depth profile will produce a continuous curvature on the surface of a post-operative cornea, where the curvature will be continuous throughout the transition zone thereby minimizing curvature discontinuities.
 2. The method of claim 1, wherein the transition zone is larger than or equal to a conventional transition zone.
 3. The method of claim 1, wherein more tissue is removed than in a conventional transition zone to facilitate reducing the biomechanical response.
 4. The method of claim 1, wherein said measuring a curvature of a pre-operative cornea includes receiving perturbation data concerning the pre-operative cornea on which the refractive ophthalmic treatment will be performed.
 5. The method of claim 4, wherein the perturbation data is both pre-perturbation and post-perturbation data, and comprises at least one of a topographic data, a pachymetric data, an elevation data, a total corneal thickness data, a corneal curvature data, a wave-front data, and an intraocular pressure data.
 6. The method of claim 5 wherein perturbation comprises one of a corneal incision, a corneal ablation, a LASIK flap cut, an ultrasonic measurement, and peeling the epithelial layer from the cornea.
 7. A computer readable medium storing computer executable instructions operable to perform the method of claim
 1. 8. A system for customizing a transition zone of an ablation pattern for a refractive ophthalmic treatment for a cornea, comprising: a data receiver for receiving a corneal data; and a transition zone designer adapted to produce the customized transition zone with a continuous curvature which eliminates curvature discontinuities at or near the edge of a post-operative optical zone and whose effects minimizes the biomechanical response in the post-operative cornea.
 9. The system of claim 8, where the corneal data comprises at least one of topographic data, pachymetric data, elevation data, total corneal thickness data, corneal curvature data, wave-front data, and intraocular pressure data, said corneal being measured before a cornea is processed by at least one of a cut, an ablation, an ultrasonic measurement, and a peeling of a corneal epithelial layer.
 10. The system of claim 8, where the corneal data comprises at least one of topographic data, pachymetric data, elevation data, total corneal thickness data, corneal curvature data, wave-front data, flap thickness data, and intraocular pressure data, said corneal data being measured after a cornea is processed by at least one of a cut, an ablation, an ultrasonic measurement, and a peeling of a corneal epithelial layer.
 11. A computer readable medium storing computer executable components of the system of claim
 8. 12. A method to facilitate an increased functional optical zone with a customized transition zone pattern of continuous curvature, where the corrective properties of the transition zone are included in the ablation zone pattern design, said method comprising: receiving pre-operative data concerning a cornea on which a refractive ophthalmic treatment will be performed; subtracting the programmed optical zone correction from corneal measurements provided in the pre-operative data to provide the predicted location of the post-operative optical zone edge; calculating the predicted curvature at and/or near the edge of the optical zone after application of the programmed optical zone correction; calculating based, at least in part, on the pre-operative data received and the predicted curvature at the edge, a customized transition zone pattern which addresses curvature discontinuity by eliminating its occurrence in and/or near the programmed optical zone; and applying the calculated transition zone to the ablation zone pattern.
 13. The method of claim 12 wherein said pre-operative data, in part, is used to determine a programmed optical zone correction used in the ablation zone pattern.
 14. The method of claim 12 wherein said pre-operative data includes, at least one of topographic data, pachymetric data, elevation data, corneal thickness data, corneal curvature data, wave-front data, and intraocular pressure data, where such data is associated with the cornea before and/or after perturbation.
 15. The method of claim 14 wherein said perturbation comprises one of a corneal incision, a corneal ablation, a LASIK flap cut, an ultrasonic measurement, and peeling the epithelial layer from the cornea.
 16. The method of claim 12 wherein said method uses a curve fitting algorithm to generate a transition zone with a continuous second derivative along a profile of the cornea outwardly from the programmed optical zone correction.
 17. The method of claim 16 wherein said curve fitting is selected from the group comprising one of spline fitting, arc-step fitting, least-squares fitting, and non-linear least squares fitting.
 18. The method of claim 12 further comprises receiving post-perturbation data which includes, at least one of topographic data, pachymetric data, elevation data, corneal thickness data, corneal curvature data, wave-front data, and intraocular pressure data, where such data is associated with the cornea after perturbation.
 19. The method of claim 18 wherein said perturbation comprises one of a corneal incision, a corneal ablation, a LASIK flap cut, an ultrasonic measurement, and peeling the epithelial layer from the cornea.
 20. The method of claim 12 further comprises taking corneal measurements, which are taken by methods including, but not limited to, corneal topography, optical coherence tomography, ultrasound, refraction, and/or wave-front analysis. 